You are here

Q: How far underground will you store CO2?

The geologic reservoirs where CO2 will be stored are nearly a mile below the surface. That’s really deep. To give you some perspective, a mile is:

5,280 feet

or

1.6 kilometers

or

the height of 34 Statues of Liberty stacked on top of each other

or

over 500 times deeper than an average swimming pool

or

roughly half the width of Manhattan

keep going . . .

Want to Read More?

A. Close to a kilometer, but more often over 2 miles deep.

Why so deep?

The major reason for storing CO2 so deep is that it's required by law. The US Safe Drinking Water Act orders that underground injection is below and isolated from freshwater aquifers. So, carbon dioxide is stored much deeper than most aquifers.

Second, carbon dioxide is most efficiently stored underground in a dense phase (sometimes referred to as supercritical carbon dioxide or scCO2 by chemists). In this dense phase, carbon dioxide has properties of both a liquid and a gas: it is easier to inject into the reservoir than a liquid and once in the reservoir it takes up less space than a gas. This dense phase of carbon dioxide occurs at pressures above 1070 psi (or 73 atm).

Conditions deep underground are very different from the conditions on Earth's surface. The pressures are much higher, just what's needed to keep CO2 in its dense phase. If you could go swimming one mile deep in the ocean, the weight of the water above you would squash the air in your lungs. This is called hydrostatic pressure. Underground, rock formations are saturated with water. The increased weight of water pushing down from above increases the hydrostatic pressure by 43.3 psi with each 100 feet. By the time you reach 2200 feet below the water table (2/3 of a kilometer or 0.4 miles deep), there is so much hydrostatic pressure that carbon dioxide remains in its dense phase.

In Texas and along the Gulf of Mexico, the reservoirs with the best geological characteristics for sequestering carbon dioxide are found even deeper, at depths of about 3.2 km, or 2 miles.

Just how geologists determine what geological formations are best for storing carbon dioxide is an area of active research. The Gulf Coast Carbon Center has developed an atlas of geological formations that have the proper characteristics for carbon storage. Click the icon left to link to the atlas.

For a visual representation of how far down CO2 is stored, take a long scroll down this webpage developed by the Midwest Geological Sequestration Consortium.

For more on state changes in carbon dioxide, including its supercritical or dense phase, click the icon at the left.

A monitoring program was recently concluded the SACROC oil field in west Texas where carbon dioxide has been injected since the 1970's to retrieve oil from mature fields. The tesing showed no effects on freshwater after 35 years of carbon dioxide injection. This is reassurance that the injection was performed in a manner that effectively isolated carbon dioxide from water reservoirs. Click on the icon at the left to read more.